(1) Field of the Invention
This invention relates to ceramic materials for thermal barrier coatings applied to metallic parts wherein an interfacial layer of stabilized zirconia is interposed between the part and the coating.
(2) Description of the Related Art
Gas turbine engines are well developed mechanisms for converting chemical potential energy, in the form of fuel, to thermal energy and then to mechanical energy for use in propelling aircraft, generating electric power, pumping fluids etc. At this time, the major available avenue for improved efficiency of gas turbine engines appears to be the use of higher operating temperatures. However, the metallic materials used in gas turbine engines are currently very near the upper limits of their thermal stability. In the hottest portion of modern gas turbine engines, metallic materials are used at gas temperatures above their melting points. They survive because they are air cooled. But providing air cooling reduces engine efficiency.
Accordingly, there has been extensive development of thermal barrier coatings for use with cooled gas turbine aircraft hardware. By using a thermal barrier coating (TBC), the amount of cooling air required can be substantially reduced, thus providing a corresponding increase in efficiency. One common TBC utilized to protect gas engine turbine parts comprises 59 weight percent Gd2O3−41 ZrO2. While providing low thermal conductivity, such Gd-Zr based TBCs may exhibit lower spallation resistance than conventional yttria stabilized zirconia, e.g. 7YSZ. It is believed that this susceptibility to spallation arises from the lower fracture toughness characteristic of Gd-Zr systems.
In response, it has been found that incorporating an initial, thin layer (nominal 0.5-1 mil) of a different stabilized zirconia, e.g., 7YSZ enhances the spallation resistance of Gd—Zr systems. While not fully understood, such increased spallation resistance likely arises from higher fracture toughness of the 7YSZ, allowing it to resist the stresses that develop at the TBC/bond coat (or more particularly the alumina layer) interface. Another possible beneficial effect of the 7YSZ interlayer is likely related to the negation of the potentially detrimental interaction between Gadolinia and the Alumina scale that forms on the surface of the substrate alloy or bond coat. Regardless of the mechanism at work, it has been shown that the addition of a thin 7YSZ interlayer compensates for the lower fracture toughness of the Gd—Zr based TBC so as to enhance spallation resistance.
In addition to spallation resistance, there is also a need to produce a TBC which exhibits resistance to erosion. Erosion occurs when fine particulates ingested or liberated by an engine impact the TBCs at very high velocity during engine operation. This results in attrition of the TBC from its surface downward. Typically, only very small particles of TBC are eroded away with a given impact event, since only fine particles tend to make it into the turbine, as large particles are centrifuged out in the compressor. Such erosive events liberate tiny chunks of TBC per event, locally reducing the thickness of the TBC slightly. Lower thermal conductivity TBCs (such as 59 GdZr) which exhibit lower fracture toughness are prone to erosion.
However, while Gd-Zr based TBCs, in particular 59weight percent Gd2O3−41 ZrO2, exhibit relatively low coefficients of thermal conductivity, there is a need for TBCs which exhibit even lower thermal conductivity. Such TBCs may exhibit less spallation and erosion resistance than do current systems. Such resistance to spallation would ideally manifest itself in both a resistance of the TBC to separate from the underlying part as well as a resistance for different layers comprising a TBC to separate one from another.
Generally speaking, metallic materials have coefficients of thermal expansion which exceed those of ceramic materials. Consequently, one of the problems that must be addressed in the development of successful thermal barrier coatings is to match more closely the coefficient of thermal expansion of the ceramic material to the metallic substrate so that upon heating, when the substrate expands, the ceramic coating material does not crack. Zirconia has a high coefficient of thermal expansion and this is a primary reason for the success of zirconia as a thermal barrier material on metallic substrates.
Despite the success with the current use of electron beam physical vapor deposited zirconia base coatings, there is a continuing desire for improved coatings which exhibit superior thermal insulation capabilities, especially those improved in insulation capabilities when normalized for coating density. Weight is always a critical factor when designing gas turbine engines, particularly rotating parts. Ceramic thermal barrier coatings are not load supporting materials, and consequently they add weight without increasing strength. There is a strong desire for a ceramic thermal barrier material which adds the minimum weight while providing the maximum thermal insulation capability. In addition, there are obviously the normal desires for long life, stability, economy etc.
What is therefore needed is a coated part comprising a thermal barrier coating offering lower thermal conductivity but which exhibits suitable resistance to spallation.